Composition containing calcium carbonate particles dispersed in sulfur for removing nitrate nitrogen

ABSTRACT

This invention relates to a denitrifying composition which is a material to be used for decomposing nitrates nitrogen in effluent by sulfur-oxidizing bacteria that consume sulfur and carbonate as nutrients and is characterized by containing particles of calcium carbonate dispersed in sulfur. Preferably, the composition contains 10 parts by weight of sulfur coexisting with 10-15 parts by weight of calcium carbonate and 1-3 parts by weight of a microporous substance. This denitrifying composition can be prepared by heating powder of calcium carbonate and sulfur thereby melting the sulfur, dispersing the powder of calcium carbonate in liquid sulfur and solidifying the dispersion by rapid cooling. The composition simultaneously contains nutrients and alkali source and hence enables denitrification to proceed stably without addition of other components.

This application is the national phase under 35 U.S.C § 371 of PCTInternational Application No. PCT/JP99/05226 which has an Internationalfiling date of Sep. 24, 1999, which designated the United States ofAmerica and was published in English.

FIELD OF TECHNOLOGY

This invention relates to a denitrifying composition for microbiallyremoving nitrates nitrogen and also to a process for producing the same.The denitrifying composition is used for purifying water or as asubstrate for culture medium in cultivation of microorganisms.

BACKGROUND TECHNOLOGY

Among technologies for removing nitrates nitrogen intended for thepurification of water, heterotrophic denitrification with the use ofmethanol or an organic carbon source in sludge as a hydrogen donor hasbeen known. This a process is influenced by decomposing systems otherthan the target microorganisms and its denitrifying efficiency persubstrate is low; however, it is capable of removing nitrates nitrogenrelatively simply and is put to practical use in those treatingfacilities which are strictly controlled not to discharge methanol orsludge to outside after its use in denitrification. That is, thedenitrifying operation according to this process cannot be completed bymerely passing water to be treated through a single denitrifying tankand requires many treating steps and tanks and facilities.

In consequence, it has been difficult to apply this process to effluentof underdrains in farms; the effluent in question contains nitrogenouscomponents mostly consisting of inorganic nitrates nitrogen and theconcentration of nitrates nitrogen has become an issue in recent years.Moreover, there is the possibility of newly generating water pollutionunless a denitrifying substrate is injected precisely in an amountconforming to the flow rate of the effluent under treatment and to theamount of nitrates nitrogen therein contained and the operation ofheterotrophic denitrification requires equipment and full-time carefulcontrol such as installed and practiced in a sewage treating plant.

In contrast, autotrophic denitrification with the use of sulfur or asulfur compound is limited to denitrification by sulfur-oxidizingbacteria; hence, it is effected at a high denitrifying efficiency persubstrate and produces sulfate ions which are not limiting factors ofwater quality. Thus, the process offers an advantage that, where thecontent of sulfate ions is 1% or less, the treated water can bedischarged as it is after control of its pH by a calcium compound. Anexample of sulfur-oxidizing bacteria is Thiobacillus denitrificans and areaction represented by the following equation is known.1.114S+NO₃ ⁻+0.669H₂O+0.337CO₂+0.0842HCO₃⁻+0.0842NH₄→0.0842C₅H₇NO₂+0.5N₂+1.114SO₄ ²⁻+1.228H⁺A number of processes for autotrophic denitrification with the use ofsulfur or a sulfur compound have been proposed, for example, in thefollowing patent literature; JP (Japan Patent) 62-56798 B (1987), JP63-45274 B (1988), JP 60-3876 B (1985), JP 01-31958 B (1989), JP 04-9119B (1992), JP 04-74598 A(1992), JP 04-151000 A (1992), JP 04-197498A(1992), and JP 06-182393 A (1992).

Of the aforementioned processes, those described in JP 62-56798 B (1987)and JP 63-45274 B (1988) have been developed for treating effluentcontaining a variety of nitrogen compounds and sulfur compounds andrespectively require a pretreatment at pH 3 or less or a step forcultivating activated sludge seeded with a group of sulfur-oxidizingbaceria as dominant. In consequence, they cannot be said to be efficientin case the principal target of denitrification is nitrates nitrogen.

The processes described in JP 60-3876 B (1985) and JP 01-31958 B (1989),although not limited to denitrification aimed at nitrates nitrogen,respectively carry out denitrification by sulfur-oxidizing bacteria inthe denitrification step following the nitrification step. However, theycannot carry out autotrophic denitrification efficiently because asulfur component needs to be added in an amount conforming to that ofnitrates nitrogen in the object to be treated or minute bubbles ofnitrogen gas produced by denitrification cannot discharge by themselvesand their discharge separately requires an aeration tank.

The process described in JP 04-9119 B (1992) relates to simultaneousremoval of nitrogen and phosphoric acid from effluent with the use ofmarble composed of calcium carbonate and sulfur particles. However, theparticles of marble and sulfur are not used in the same step and theprocess is fundamentally an aerobic-anaerobic activated sludgetreatment. Therefore, unlike autotrophic denitrification with sulfuralone as denitrifying substrate, the process requires control of sludgeand is not efficient for direct denitrification of nitrates nitrogen.

The process described in JP 04-74698 A (1992) is fundamentally ananaerobic-aerobic activated sludge treatment, but it performs morestably in removal of nitrates nitrogen than the aforementioned processesbecause of the introduction of sodium hydrogen carbonate or calciumcarbonate as carbon source. The process, however, is not efficient fordirect denitrification of nitrates nitrogen since it is low indenitrifying efficiency on account of pyrites being used as sulfursource and, besides, it remains in the category of activated sludgeprocesses requiring a number of treating tanks.

The process described in JP 04-151000 A (1992) relates to autotrophicdenitrification by sulfur-oxidizing bacteria with sodium hydrogencarbonate or calcium carbonate supplied as carbon source and athiosulfate salt used as sulfur source and electron donor. Thethiosulfate salt must be injected in an amount conforming to that ofnitrates nitrogen in the object to be treated and, like theaforementioned, the process is not efficient for direct denitrificationof nitrates nitrogen.

The process disclosed in JP 04-197498 A (1992) relates to autotrophicdenitrification by sulfur-oxidizing bacteria as a pretreatment inpurification of water. In this case as well, the efficiency dropsmarkedly unless sodium sulfite is added in an amount conforming to thatof the object of denitrification contained in raw water and the processcannot be adapted easily to direct denitrification of nitrates nitrogen.

Any one of the processes described thus far does not aim at directdenitrification of nitrates nitrogen as its object and most of them areno better than activated sludge processes and inefficient from thestandpoint of denitrification. On the contrary, the process described inJP 06-182393 A(1994) can remove nitrate nitrogen efficiently bysulfur-oxidizing bacteria.

This process, however, uses sulfur powder that is highly reactive towardsulfur-oxidizing bacteria and effects denitrification by passing rawwater containing nitrates nitrogen and the like through a fluidized bedreactor vessel in which a layer filled with sulfur powder is formed.Motive power is necessary to release nitrogen gas adhering to andbetween the sulfur particles in the course of denitrification and,unless the gas is released, the sulfur particles exhibiting gooddenitrifying efficiency become wrapped in bubbles and cannot participatein denitrification any longer. The same is true for the bubbles betweenparticles. Other difficulties are a necessity for separate correction ofthe acidity of sulfuric acid being formed and a necessity for forcedpassage of raw water at all times to prevent the occurrence of highacidity which would stop the denitrification. Thus, general applicationof this process, for example, to the treatment of effluent ofunderdrains would be difficult to realize because it would incurconsiderable capital and running costs.

In addition, the aforementioned processes do not provide any concretemeasures for supply of carbon source which is just as important assulfur source and for control of the pH, namely, keeping the pH in thevicinity of 7, in order to maintain the activity of sulfur-oxidizingbacteria and effect denitrification with certainty.

Recent knowledge obtained in a learned society for water treatmentsuggests the importance of taking the following actions in order toeffect denitrification smoothly by sulfur-oxidizing bacteria.

That is, it is essential to accelerate the multiplication ofsulfur-oxidizing bacteria in order to sustain the denitrificationreaction by the bacteria in question and, to accomplish this, it isnecessary to supply with certainty carbon source that is required forthe synthesis of bacteria in addition to sulfur that is a source ofnutrient and to control the pH generally at 7 or higher to maintain themicrobial activity. Reference should be made to (4-28) Removal ofNitrate Nitrogen from Actual Sewage by Denitrification with Sulfur, the49th National Meeting for Presentation of Researches on Water Supply,May, 1998, Abstracts of Papers, pp. 238-239 (4. Section of WaterPurification); Japan Water Supply Society. However, it is not alwaysnecessary to keep the pH at 7 or more as will be described below.

As illustrated above, the most efficient way for safe removal ofnitrates nitrogen is autotrophic denitrification by sulfur-oxidizingbacteria, but it has been difficult to produce surely and easily theeffect of this process.

Accordingly, an object of this invention is to provide a denitrifyingcomposition for microbially removing nitrates nitrogen which does notrequire supply of carbon source that is essential to microorganisms,exhibits a good balance in the system before and after the reaction,reduces the influences of pH and chemical substances to the water area,and performs stably.

Another object of this invention is to provide a denitrifying materialwhich can be brought into contact with effluent even during a dearth ofwater and can maintain a high denitrifying efficiency at all times.

A further object of this invention is to provide a process for producingthe aforementioned denitrifying composition in a short time at low cost.

DISCLOSURE OF THE INVENTION

Thus, this invention relates to a denitrifying composition formicrobially removing nitrates nitrogen comprising particles of calciumcarbonate dispersed in sulfur. This invention also relates to adenitrifying composition comprising particles of calcium carbonate andparticles of a substance possessing a cation exchange capacity or amicroporous substance dispersed in sulfur. This invention furtherrelates to a denitrifying material comprising a mixture of theaforementioned denitrifying composition and mineral fibers. Thisinvention still further relates to a process for producing adenitrifying composition which comprises heating powder of calciumcarbonate and sulfur thereby melting the sulfur and dispersing thepowder of calcium carbonate in the liquid sulfur and solidifying thedispersion by rapid cooling.

A denitrifying composition of this invention comprises a substancemainly consisting of calcium carbonate and sulfur as main components.Moreover, a denitrifying composition of this invention is a granular ormassive substance comprising a substance mainly consisting of calciumcarbonate and sulfur as main components and a microporous substance as acoexisting component; more particularly, a denitrifying composition ofthis invention is a granular or massive substance comprising 10-15 partsby weight of a substance mainly consisting of calcium carbonate and 10parts by weight of sulfur as main components and 1-3 parts by weight ofa microporous substance as a coexisting component. Still more, adenitrifying composition of this invention is a granular or massivesubstance comprising a substance mainly consisting of calcium carbonateand sulfur as main components and a microporous mineral product and/or aprocessed product thereof as a coexisting component; more particularly,a denitrifying composition of this invention is a granular or massivesubstance comprising 10-15 parts by weight of a substance mainlyconsisting of calcium carbonate and 10 parts by weight of sulfur as maincomponents and 1-3 parts by weight of a microporous mineral productand/or a processed product thereof as a coexisting component.Furthermore, a denitrifying composition of this invention is a granularor massive substance comprising a substance mainly consisting of calciumcarbonate and sulfur as main components and a microporous carbide as acoexisting component; more particularly, a denitrifying composition ofthis invention is a granular or massive substance comprising 10-15 partsby weight of a substance mainly consisting of calcium carbonate and 10parts by weight of sulfur as main components and 1-3 parts by weight ofa microporous carbide as a coexisting component. Still further, adenitrifying composition of this invention is a granular or massivesubstance comprising a substance mainly consisting of calcium carbonateand sulfur as main components and a microporous mineral product and/or aprocessed product thereof and a microporous carbide as coexistingcomponents; more particularly, a denitrifying composition of thisinvention is a granular or massive substance comprising 10-15 parts byweight of a substance mainly consisting of calcium carbonate and 10parts by weight of sulfur as main components and 1-3 parts by weight ofa microporous mineral product and/or a processed product thereof and amicroporous carbide as coexisting components. Finally, a denitrifyingcomposition of this invention is a granular or massive substancecomprising 10-15 parts by weight of calcium carbonate and 10 parts byweight of sulfur as main components and 1-3 parts by weight ofkieselguhr and/or carbon derived from rice hull as coexistingcomponents.

Since a denitrifying composition of this invention contains nutrientsfor sulfur-oxidizing bacteria which act to remove nitrates nitrogen andis consumed as denitrification progresses, it may be called either acomposition for giving microbial activity or a substrate for removingnitrates nitrogen. Here, the nitrates nitrogen includes nitrates ionsand nitrite ions.

A denitrifying composition of this invention contains calcium carbonateand sulfur as essential components; concretely, there exist acomposition comprising calcium carbonate (including a substance mainlycomprising of calcium carbonate) and sulfur (including a substancemainly comprising of sulfur) and a composition comprising calciumcarbonate and sulfur as main components and a microporous substance as acoexisting component.

Calcium carbonate or a substance mainly consisting of calcium carbonatecontained in a denitrifying composition of this invention is exemplifiedby calcium carbonate, powdered or crushed limestone, powdered or crushedseashell, powdered or crushed fossil of seashell, powdered or crusheddolomite, and powdered or crushed coral and they may be used eithersingly or as a mixture of two kinds or more.

Calcium carbonate or a substance mainly consisting of calcium carbonateto be used for producing a denitrifying composition of this invention isprepared as powder with a specific surface area of 2,000-5,000 cm²/g,preferably 2,500-4,000 cm²/g, by air permeability determination. Thedispersibility to sulfur decreases if the specific surface area is toolarge or too small. An average particle size of less than 1 mm isbeneficial to good dispersibility.

Sulfur contained in a denitrifying composition of this invention ispreferably amorphous sulfur that is obtained by rapidly cooling moltensulfur. Sulfur to be used for producing a denitrifying composition ofthis invention is under no restriction as long as it can be melted andsolidified by rapid cooling; for example, sulfur recovered in a plantfor desulfurizing petroleum or coal and natural sulfur are useful,either as solid such as powder and granule or molten, and such sulfurmay contain contaminants.

A microporous substance contained in a denitrifying composition of thisinvention is any substance with pores suitable for sulfur-oxidizingbacteria to settle in, preferably a mineral product or a processedproduct thereof and a carbide. Mineral products or processed productsthereof include kieselguhr, calcined kieselguhr, tuff, kokaseki,perlite, pearlite, porous ceramic, brick, ALC, pumice stone, pozzolan,shirasu, shirasu balloon, calcined expanded shale, attapulgite,sepiolite, cristobalite, sericite, acid clay, and illite. Carbides arecarbonaceous materials such as charcoal and include charcoal, carbonderived from coconut shell, carbon derived from rice hull, coke, carbonderived from bamboo, and activated carbon. Other microporous substancesinclude volcanic ashes, soil, fly ash, cement, and concrete.

The water content in a microporous substance to be used in theproduction of a denitrifying composition of this invention is preferably30% or less while the particle size is preferably 5 mm or less, morepreferably 1 mm or less, in length. However, carbon derived from ricehull, uncaked shirasu, uncaked volcanic ashes, and uncaked soil may beused with no particular adjustment of the particle size.

A microporous sustance contained in a denitrifying composition of thisinvention may advantageously be a cation exchanger. Such cationexchangers include natural zeolites, synthetic zeolites, and bentonite.A cation exchanger, when substituted for a microporous substance, canremove ammoniacal nitrogen from effluent by adsorption and, besides, canhold the negatively charged microorganisms in the denitrifyingcomposition by adsorbing calcium ions produced by the denitrificationreaction. If necessary, it is possible to add sand, slag, and others toa denitrifying composition of this invention to the extent that thefunction of the composition is not adversely affected.

The ratio of sulfur to calcium carbonate in a denitrifying compositionof this invention is not restricted as long as the-sulfur works as abinder or matrix and gives a solid with a certain degree of strength,but the ratio by weight of sulfur to calcium carbonate is preferably 3:1to 1:3, more preferably 2:1 to 1:2. Both sulfur and calcium carbonateserve as nutritents and gradually dininish and it is not advantageous toset the ratio in such a manner as to markedly destroy the balance of thetwo. The amount of sulfur is kept in the range 25-75%, preferably33-67%, of the whole even in the cases where a microporous substance andothers are added. A more preferable proportion is 10 parts by weight ofsulfur, 10-15 parts by weight of calcium carbonate or a substance mainlycomprising thereof and 1-3 parts by weight of a microporous substance.Where a cation exchanger is used as a microporous substance, zeolite issubstituted to an amount corresponding to 5-20%, preferably 10% or so,of calcium carbonate while bentonite is substituted to an amountcorresponding to 1-5%, preferably 2% or so, of calcium carbonate.

There is no particular restriction to the shape of a denitrifyingcomposition of this invention, but it is advantageous for thecomposition to have a certain size and a surface area as large aspossible in order to increase the contact area with raw water oreffluent to be treated and to prevent the composition itself fromflowing out. Therefore, a preferable shape is massive, granular, ormolded. The molded form here means a form with a specific shape such asplate, bar, and honeycomb. In case of a mass, a particle diameter of0.25-1 mm produces the anticipated effect faster than those with aparticle diameter of 1-3 mm or 3-5 mm, but a smaller diameter has alarger possibility of blocking and flow-out loss. Hence, the optimalsize varies with the mode of usage. The average particle diameter for along-term industrial use is adequately 2-50 mm and is advantageouslykept below 100 mm.

A denitrifying composition of this invention is produced as follows:where sulfur, calcium carbonate or a substance comprising mainlythereof, and a microporous substance are used, they are mixed, thesulfur is melted by heating at 112-180° C., preferably 112-125° C., themixture is rapidly cooled, for example, by throwing it into water, andthe resulting solid is crushed or granulated.

Instead of melting sulfur by heating, sulfur melted in advance may beused. That is, sulfur is melted first, calcium carbonate, a microporoussubstance, and other substances to be added as needed are added, mixed,and rapidly cooled. During this operation, it is preferable to addcalcium carbonate, a microporous substance, and other substances in theform of powder or granule as mentioned earlier. Here, the specificsurface area of calcium carbonate in use is 2,000-5,000 cm²/g,preferably 2,500-4,000 cm²/g, by air permeability determination. Alarger particle size is more advantageous where calcium carbonate isused in a larger amount and calcium carbonate with a particle diameterof 5 mm or so can be used up to approximately three times the amount ofsulfur.

Calcium carbonate with a smaller particle diameter is more effective forenhancing the activity of microorganisms, but a specific surface area inexcess of 5,000 cm²/g by air permeability determination decreases thebulk specific gravity and it becomes difficult for calcium carbonate tocoexist in an amount required for maintaining the pH at a neutral level.With a specific surface area of 2,500-4,000 cm²/g, it is possible toobtain not only good granules or masses but also the coexistence ofcalcium carbonate and sulfur at a weight ratio of approximately 1:1. Theratio of calcium carbonate to sulfur is 30-300 parts by weight,preferably 100-150 parts by weight, more preferably 100-120 parts byweight, of calcium carbonate to 100 parts by weight of sulfur, and mostpreferably the two are used in approximately the same amount. A smalleramount of calcium carbonate causes a shortage of calcium carbonatenecessary for the neutralization reaction while a larger amount causes ashortage of the ability to act as a binder of sulfur and makes itdifficult to obtain a stable and strong solid.

For incorporation of a microporous substance, 10 parts by weight ofsulfur, 10-15 parts by weight of calcium carbonate, and 1-3 parts byweight of a microporous substance are mixed, the sulfur is melted toform a homogeneous dispersion, and the dispersion is solidified by rapidcooling.

After solidification by rapid cooling, the solid is crushed to masses orgranules or further processed to molded forms. Crushing renews thesurface and exposes faces other than the sulfur layer with the resultantincrease in performance as denitrifying material.

The solid obtained by this invention can be crushed or granulated in theusual manner and a special granulating step such as pressing is notrequired. Moreover, when the solid obtained by this -invention is socrushed as to conform to the target particle diameter, the whole surfaceformed by crushing acts effectively to let microorganisms manifest theiractivity and, while not in use, is oxidized with difficulty by oxygen inair and exhibits good storage stability.

A denitrifying material of this invention comprises a denitrifyingcomposition of this invention and mineral fibers. Mineral fibers to bemixed with a denitrifying composition include rock wool, glass wool,ceramic wool, and carbon wool, either singly or as a mixture of twokinds or more, and inexpensive rook wool is preferable. Rock wool isreadily processed to granular products, exhibits excellent waterretention, has openings suitable for the growth of microorganisms, andperforms a function of neutralizing even highly acidic effluent becauseof its basic chemistry.

Rock wool is obtained by melting a variety of metallurgical slags suchas blast furnace slag and electric furnace slag, natural rocks such asbasalt and diabase, or a mixture of the foregoing in an electric furnaceor a cupola and converting the melt to fibers by application ofcentrifugal force and/or pressurized gas. Rock wool is mainly composedof CaO, SiO₂ and Al₂O₃ and additionally contains MgO, Fe₂O₃ and thelike. Granular rock wool is obtained by processing rock wool by agranulator and it is useful when its particle diameter is 1-50 mm,preferably 5-20 mm. Also useful is a material obtained by molding amixture of rock wool and a resin binder into a board followed by cuttingor crushing or a material obtained by solidifying a mixture of granularrock wool and an inorganic hydraulic binder. Rock wool of this type canbe mixed as it is with a denitrifying composition or, if necessary,after preparation of particles of uniform size by such operation asclassification.

Mixing of a denitrifying composition and mineral fibers can be carriedout by preparing the two in the desired shape and size by such means ascrushing and then blending the two in a known mixer such as drum tumblerand ribbon blender. The mixing ratio of the two is 100 parts by weightof the denitrifying composition and 5 parts by weight or more,preferably 10-500 parts by weight, of the mineral fibers. A smalleramount of mineral fibers leads to a lower ability of the denitrifyingcomposition to retain water and the portion of the denitrifyingcomposition protruding from the surface of the water during a dearth ofwater becomes dry.

Mixing of mineral fibers in this manner allows a denitrifying materialto maintain its ability to retain water even during a dearth of water.That is, water is sucked up from the mineral fibers in water to thoseabove the surface of the water by capillary action and a denitrifyingmaterial containing a denitrifying composition and protruding above thesurface of the water does not dry even during a dearth of water andsulfur-oxidizing bacteria do not perish. Effluent sucked up into thedenitrifying material above the surface of the water is alsodenitrified.

Rock wool is rich in mineral ingredients such as Mn, Zn, Cu, Mo, Fe, andB and they elute and produce an effect to activate sulfur-oxidizingbacteria. Moreover, a large volume of openings present in mineral fibersis suitable for the growth of other microorganisms such asammonia-assimilating bacteria and protozoa that decompose organicmatters.

A denitrifying composition obtained in this manner and a denitrifyingmaterial containing the composition are suitable for purifying waterrelating to nitrates nitrogen while removing nitrates nitrogen of highconcentration and preventing the pH of treated water from becomingstrongly acidic.

The reason for this beneficial effect of a denitrifying composition ofthis invention is the coexistence of calcium carbonate and sulfur in thesame particle. Furthermore, the additional coexistence of a microporoussubstance simultaneously provides micropores for a carrier or a dwellingof sulfur-oxidizing bacteria or for a place of colonization and growthof bacteria. Calcium carbonate and sulfur acting as nutrients make itunnecessary to supply from outside sulfur that is a source of nutrientand carbon that is necessary for the synthesis of bacteria and theactive region of sulfuroxidizing bacteria is nearly neutral in pH wherea high microbial activity is maintained. With a composition such asthis, the pH is measured nearly neutral and ions are in good balance.Hence, the denitrifying ability improves markedly and contributes agreat deal to improvement of water quality; for example, nitratesnitrogen present even at high concentration exceeding 150 ppm instagnant water can surely be removed.

The aforementioned situation is explained with reference to FIG. 1wherein calcium carbonate (CaCO₃) is dispersed in sulfur (S). Amicroporous substance is also present dispersed in sulfur (S).Sulfur-oxidizing bacteria dwell not only near the surface of thedenitrifying composition but also inside the microporous substance andtheir principal active region is near the surface of the denitrifyingcomposition This active region which is in contact with the water to betreated is nearly neutral.

In contrast, in accordance with the method for mixing individualparticles of calcium carbonate and sulfur as shown in FIG. 2, the pH ofthe treated water is seemingly neutral and the resulting balance of ionsis seemingly good, but the region near the surface of a sulfur (S)particle which is the active region of sulfur-oxidizing bacteria becomesacidic and the microbial activity diminishes. Also, the region near thesurface of a calcium carbonate (CaCO₃) particle becomes alkaline.

A denitrifying composition of this invention can stably performdenitrification at all flow velocities excepting the limiting flowvelocity. Moreover, calcium carbonate in coexistence is in an amountsufficiently reactive to neutralize the produced sulfuric acid and thesystem does not turn alkaline nor acidic and affects the water areainsignificantly. Furthermore, negatively charged microorganisms can beretained as a result of adsorption of ammoniacal nitrogen by addition ofa substance possessing an ion exchange capacity and adsorption ofcalcium ions effected separately. It is also possible to let phosphoricacid adhere to a part of calcium carbonate and let plants utilize suchadhering phosphoric acid. For example, with the use of a denitrifyingcomposition of this invention, it is possible to promote purification ofwater by applying a yellow iris with a denitrifying composition of thisinvention used as culture medium.

A denitrifying composition and a denitrifying material of this inventioncan be used for treating effluent containing nitrates nitrogen, forexample, treating factory effluent, city sewage, and agriculturaleffluent such as effluent of underdrains in farms. The composition orthe material may be used in a variety of manners, for example, it isplaced in a cage or a net and immersed in effluent, a column is packedwith it and effluent is passed through the packed column, or it isdispersed in a tank and brought into contact with effluent; however, itis advantageous to immerse it in the channels for effluent ofunderdrains and use it over a long period of time. Continuous use forseveral years is feasible if the target is effluent of relatively lowconcentration of nitrates nitrogen such as agricultural effluent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the surface and inner structure ofa denitrifying composition of this invention.

FIG. 2 is a schematic illustration of the surface and inner structure ofa denitrifying composition prepared by merely mixing the particles ofcalcium carbonate and sulfur.

PREFERRED EMBODIMENTS OF THE INVENTION EXAMPLE 1

In a vat were placed 10 parts by weight of calcium carbonate powder(specific surface area, 3,100 cm²/g) and 10 parts by weight of sulfur,the contents were heated at approximately 120° C. to melt the sulfur,and mixed homogeneously by stirring. The mixture was then thrown into alarge volume of water at normal temperature for rapid cooling,solidified, and crushed to prepare a denitrifying composition (invention4) with a particle diameter of 5-10 mm.

EXAMPLE 2 AND COMPARATIVE EXAMPLE 1

A denitrifying composition (invention 1) was prepared as in Example 1 byusing 10 parts by weight of calcium carbonate powder.(specific surfacearea, 3,100 cm²/g), 10 parts by weight of sulfur, and 3 parts by weightof carbon derived from rice hull.

Likewise, a denitrifying composition (invention 2) was prepared as inExample 1 by using 10 parts by weight of calcium carbonate powder(specific surface area, 3,100 cm²/g), 10 parts by weight of sulfur, 1part by weight of carbon derived from rice hull, and I part by weight ofkieselguhr.

Additionally, a denitrifying composition (invention 3) was prepared asin Example 1 by using 10 parts by weight of calcium carbonate powder(specific surface area, 3,100 cm²/g) 10 parts by weight of sulfur, and 2parts by weight of kieselguhr.

As a comparative example, a material (comparison 1) was prepared bymerely mixing 10 parts by weight of granular sulfur with a particlediameter of 5-10 mm and 10 parts by weight of calcium carbonateparticles.

EXAMPLE 3

Each of inventions 1 to 4 and comparison 1 prepared in Examples 1 and 2and Comparative Example 1 was taken as specimen, placed in a flasktogether with test water, and batch-tested for denitrification at anaverage water temperature of 20° C. The test water was prepared byadding 1 wt % of river sediment (containing sulfur-oxidizing bacteria)sampled from underdrains in Fukaya City, Saitama Prefecture, to rawwater which was an aqueous solution (aqueous KNO₃) with a nitratenitrogen concentration of, 150 mg/1. For comparison, a test wasconducted similarly free of any of the specimens by using test watercontaining only the river sediment.

Table 1 shows a decrease in the concentration (mg/l) of nitratesnitrogen in the batch test. Any of the inventions was confirmed toremove nitrates nitrogen of high concentration satisfactorily.

TABLE 1 Starting After After After After After Specimen day 5 days 10days 15 days 20 days 30 days Invention 1 175 111 68 45 14  8 Invention 2175 111 75 52 18  10 Invention 3 175 111 79 52 19  8 Invention 4 175 12293 77 29  20 Comparison 1 175 122 122  131  80  82 None 175 173 172 172 172  172

The pH of the system changed as shown in Table 2 and the pH of thetreated water was confirmed not to turn strongly acidic.

TABLE 2 After After After After After Specimen 5 days 10 days 15 days 20days 30 days Invention 1 6.7 6.7 6.6 6.6 6.5 Invention 2 6.6 6.5 6.6 6.56.5 Invention 3 6.8 6.8 6.9 6.6 6.5 Invention 4 6.8 6.7 6.7 6.6 6.6Comparison 1 6.8 6.7 6.7 6.6 6.5 None 6.7 6.0 4.5 4.9 4.3

The concentrations in mg/l of sulfate ions and calcium ions weredetermined 20 days after the start of denitrification and the resultsare shown in Table 3.

TABLE 3 SO₄ ²⁻ Ca²⁺ Invention 1 503 523 Invention 4 495 520 Comparison 1480 540

EXAMPLE 4

Sulfur powder was melted at 120° C., mixed homogenously with lime powderat a ratio by weight of 1:1, and the mixture was cooled rapidly andcrushed to a particle diameter of 5-20 mm to prepare a denitrifyingcomposition (invention 5). A test plant was filled with 80 kg (0.94 m³)of the composition, and effluent of the following properties fromunderdrains in a farm was treated by passage through the test plant: pH,6.8-8.0; SS, 0-3 mg/l; NH₄—N, 0.01 mg/l or less; NO₃—N, 60-93 mg/l ; SO₄²⁻—S, 22-37 mg/l ; Ca²+, 90-132 mg/l . Before the start of theexperiment, 100 g of the actual soil cultivated on the Taylor culture C,medium as microorganism source was domesticated for 8 days in acontainer filled with effluent from underdrains.

When the experiment was carried out by controlling the loading rate(amount of raw water per day/amount filled of denitrifying composition)at 0.5-10, removal of 99% or more of the concentration of nitratesnitrogen in raw water was obtained at an increasingly higher loadingrate up to approximately twice per day and the treated water showed thefollowing properties: pH, 6.8-7.3; SS, 0 mg/l ; NH₄—N, 0.4-1.4 mg/l ;SO₄ ²⁻—S, 184-214 mg/l ; Ca²⁺, 170-230 mg/l . The maximum denitrifyingrate then was 207 g as nitrogen per day per 1 ton. There was nochanneling and dislocation of bubbles and a stable operation waspossible during the test period extending over approximately 6 months.

EXAMPLE 5

Sulfur powder was melted at 150° C., mixed homogeneously with heavycalcium carbonate (T-200, product of K.K. Nitchitsu, Blaine specificsurface area 2,800 cm²) at a ratio by weight of 1:1, rapidly cooled bycold water, and crushed to a particle diameter of 5-20 mm to prepare adenitrifying composition.

In a ribbon blender were placed 100 parts by weight of the denitrifyingcomposition and 20 parts by weight of granular rock wool (S-fiber,product of Shinnikka Rock Wool K.K., average particle diameter 30 mm) toprepare a denitrifying material.

The material was used in batch test for denitrifying raw water preparedby adding 50 ml of a commercial culture of sulfur-oxidizing bacteria(DSM807) to 1,000 ml of artificial effluent that was prepared by addingnitrates nitrogen to pure water at a rate of 150 mg/l.

The denitrifying test was carried out by introducing 150 ml of theaforementioned raw water to a glass vessel containing 300 g of thedenitrifying material and keeping the contents at an average watertemperature of 20° C. for 7 days. The degree of removal of nitratesnitrogen after 7 days was 100%. Thereafter, the amount of theaforementioned raw water was increased to 1,000 ml in order to immersethe portion of the denitrifying material protruding above the surface ofthe raw water and the test was continued for another 7 days. The removalof nitrogen after a total of 14 days was 100%.

EXAMPLE 6

A denitrifying material was prepared as in Example 5 except adding 10parts by weight of carbon derived from rice hull and the denitrificationtest was carried out as in Example 5. The removal of nitrogen was 100%after 7 days and it was also 100% after 7 days from the time ofincreasing the amount of the raw water or after a total of 14 days.

As is apparent from the aforementioned examples, where a denitrifyingmaterial was prepared by mixing a denitrifying composition with granularrock wool, the denitrifying acitivity of sulfur-oxidizing bacteria doesnot diminish even when the material in part protrudes above the surfaceof the raw water and the degree of denitrification as a whole can beprevented from dropping.

INDUSTRIAL APPLICABILITY

A denitrifying composition of this invention for microbially removingnitrates nitrogen performs excellently with a high degree of removal ofnitrates nitrogen because of the coexistence therein of calciumcarbonate and sulfur; the composition excludes the need to supplyessential nutrients for the microorganisms, maintains a good balance inthe system before and after the reaction and can be used directly assubstrate for cultivation of the microorganisms. Moreover, since calciumcarbonate coexists in an amount sufficiently reactive to neutralizesulfuric acid being formed, the composition turns neither alkaline noracidic, influences the water area insignificantly, and can be applied totreating effluent containing a variety of nitrates nitrogen. Adenitrifying composition of this invention which contains a substancemainly consisting of calcium carbonate and sulfur as main components anda microporous substance as a co-existing component performs excellentlyin removing nitrates nitrogen present in high concentration in effluentof underdrains in heavily manuring farms, the growth of bacteria occurssmoothly as sulfur that is nutrient for sulfur-oxidizing bacteria isoxidized as needed by the bacteria and carbon that is necessary for thesynthesis of bacteria coexist and, at the same time, the removal ofnitrogen proceeds efficiently as micropores provide places forcolonization of bacteria. Moreover, the pH is well balanced andmaintained at 6.5-7.5 in the system. With the use of a denitrifyingcomposition of this invention, water is retained during a dearth ofwater, the substrate for removing nitrates nitrogen protruding above thesurface of the water does not dry, and sulfur-oxidizing bacteria do notperish. Furthermore, the mineral components in mineral fibers areeffective for activating sulfur-oxidizing bacteria.

A process of this invention for producing the aforementioneddenitrifying composition can be practiced efficiently in a short time atlow cost.

1. A denitrifying material comprising a mixture of a denitrifyingcomposition for microbially removing nitrate nitrogen from water, saidcomposition comprising particles of calcium carbonate dispersed insulfur by heating and dispersing calcium carbonate particles in meltedsulfur and solidifying the dispersion by cooling, wherein a microporoussubstance is additionally dispersed in the sulfur and wherein the rationby weight of sulfur to calcium carbonate is 1:0.3 to 1:3, and mineralfibers.
 2. The denitrifying material of claim 1, wherein thedenitrifying composition comprises 10 parts by weight of sulfur, 10-15parts by weight of calcium carbonate, and 1-3 parts by weight of amicroporous substance.
 3. The denitrifying material of claim 1, whereinthe sulfur in the denitrifying composition is amorphous sulfur.
 4. Thedenitrifying material of claim 1, wherein the shape of the denitrifyingcomposition is granular, massive, or molded.
 5. The denitrifyingmaterial of claim 1, wherein the microporous substance in thedenitrifying composition is carbon derived from rice hull.
 6. Thedenitrifying material of claim 1, wherein the microporous substance inthe denitrifying composition is kieselguhr.
 7. The denitrifying materialof claim 1, wherein the microporous substance in the denitrifyingcomposition is a cation exchanger.
 8. The denitrifying material of claim7, wherein said cation exchanger is selected from the group consistingof natural zeolites, synthetic zeolites, and bentonite.
 9. Thedenitrifying material of claim 1, wherein said mineral fibers are rockwool.
 10. A method of decreasing the nitrate nitrogen concentration ofwater which comprises the step of contacting water containing nitrateions with the denitrifying material of claim
 1. 11. A method ofdecreasing nitrate nitrogen concentration of an effluent selected fromthe group consisting of factory effluent, sewage effluent, andagricultural effluent, which method comprises the steps of placing thedenitrifying material of claim 1 in a cage or a net to provide adenitrifying assembly and immersing the denitrifying assembly in saideffluent.
 12. A method of decreasing nitrate nitrogen concentration ofan effluent selected from the group consisting of factory effluent,sewage effluent, and agricultural effluent, which method comprises thesteps of packing a column with the denitrifying material of claim 1 toprovide a denitrifying assembly and passing said effluent through saiddenitrifying assembly.
 13. A method of decreasing nitrate nitrogenconcertration of an effluent selected from the group consisting offactory effluent, sewage effluent, and agricultural effluent, whichmethod comprises the steps of dispersing the denitrifying material ofclaim 1 in a tank and bringing said effluent into contact with saidcomposition in said tank.